Essential knowledge of optical fiber and cable (3)

41.What are the factors causing noise in optical fiber communication system?

A: there are the noise generated in the extinction ratio is unqualified, the noise of the light intensity of random change, time jitter caused by noise, receiver noise and thermal noise, the model of the fiber noise, the noise produced by the pulse broadening caused by dispersion, LD mode distribution of noise, LD frequency chirp noise and the noise reflection.

42. What are the main optical fibers used in the construction of transmission networks? What are its main features?

Answer: there are three main types, namely G, 652 conventional single-mode fiber, G, 653 dispersion displacement single-mode fiber and G, 655 non-zero dispersion displacement fiber.

The dispersion of G and 652 single-mode fiber is larger in c-band 1530 ~ 1565nm and l-band 1565 ~ 1625nm, which is generally 17 ~ 22psnm•km. When the system speed reaches above 2 or 5Gbit/s, the dispersion compensation needs to be carried out. At 10Gbit/s, the system dispersion compensation cost is larger.

The dispersion of G and 653 dispersion displacement fiber in C band and L band is generally -1 ~ 3, 5psnm•km, and zero dispersion at 1550nm. The system speed can reach 20Gbit/s and 40Gbit/s, which is the best fiber for ultra-long distance transmission of single wavelength. However, due to its zero-dispersion characteristics, nonlinear effect will occur when DWDM capacity expansion is adopted, which will lead to signal crosstalk and produce four-wave mixing FWM. Therefore, DWDM is not suitable for use.

G, 655 non-zero dispersion displacement fiber: G, 655 non-zero dispersion displacement fiber has a dispersion of 1 ~ 6psnm•km in C band, and a dispersion of 6 ~ 10psnm•km in L band. It has a small dispersion and avoids the zero dispersion zone, which not only inhibits FWM of four-wave mixing, but also can be used for DWDM capacity expansion and high-speed system opening. The new G and 655 fiber can expand the effective area to 1, 5 ~ 2 times of the general fiber, and the large effective area can reduce the power density and reduce the nonlinear effect of the fiber.

43. What is the nonlinearity of optical fiber?

Answer: it means that when the optical power of the incoming fiber exceeds a certain value, the refractive index of the fiber will be non-linear related to the optical power, and generate Raman scattering and brillouin scattering, so that the frequency of incident light will change.

44. What impact will fiber nonlinearity have on transmission?

A: nonlinear effects will cause some additional losses and interference, which will worsen the performance of the system. WDM system has large optical power and long distance transmission along optical fiber, so nonlinear distortion occurs. There are two kinds of nonlinear distortion: stimulated scattering and nonlinear refraction. The stimulated scattering includes Raman scattering and brillouin scattering. The above two kinds of scattering reduce the energy of incident light and cause loss. It can be ignored when the input power is small.

45. What is PON (passive optical network)?

Answer: PON is the optical fiber loop optical network in the local user access network, based on passive optical devices, such as couplers and light splitters

There are many reasons for optical fiber attenuation

1. The main factors causing optical fiber attenuation are: intrinsic, bending, extrusion, impurity, uneven and butt joint, etc.

Intrinsic: it is the intrinsic loss of fiber, including Rayleigh scattering, intrinsic absorption, etc.

Bending: when the fiber is bent, the light in part of the fiber will be lost due to scattering, resulting in loss.

Extrusion: loss due to slight bending of the optical fiber when it is squeezed.

Impurity: loss caused by the absorption and scattering of light in the optical fiber by impurities in the optical fiber.

Non-uniform: loss caused by non-uniform refractive index of optical fiber materials.

Docking: loss generated during fiber docking, such as: non-coaxial (single-mode fiber coaxiality requirements are less than 0, 8 microns), the end face is not perpendicular to the axis, the end face is uneven, the butt diameter is not matched and the welding quality is poor.

When light enters one end of the fiber and leaves the other, the intensity of the light decreases. This means that after the optical signal travels through the optical fiber, the light energy is partially attenuated. This means that something in the fiber, or for some reason, blocks the light signal. This is the transmission loss of optical fiber. Only by reducing fiber loss can the optical signal be smooth.

2. Classification of fiber loss

Fiber loss can be roughly divided into inherent loss of fiber and additional loss caused by service conditions after fiber is made. Specific breakdown is as follows:

Fiber loss can be divided into intrinsic loss and additional loss.

Intrinsic loss includes scattering loss, absorption loss and loss caused by imperfect fiber structure.

The additional loss includes micro-bending loss, bending loss and connection loss.

Among them, the additional loss is caused artificially in the laying process of optical fiber. In practical application, it is inevitable to connect optical fibers one by one, which will cause losses. Optical fiber tiny bending, extrusion, tensile force will also cause loss. These are optical fiber use conditions caused by the loss. The main reason is that under these conditions, the transmission mode in the fiber core changes. Additional losses can be avoided as much as possible. Next, we only discuss the intrinsic loss of optical fiber.

In the intrinsic loss, the scattering loss and absorption loss are determined by the characteristics of the fiber itself, and the intrinsic loss caused by different working wavelengths is also different. It is very important to understand the mechanism of loss and quantitatively analyze the loss caused by various factors for the development of low loss optical fiber and rational use of optical fiber.

3. Absorption loss of materials

The materials used to make optical fibers absorb light energy. After absorbing the light energy, the particles in the fiber materials vibrate and generate heat, and the energy is scattered and lost, thus the absorption loss is generated. We know that matter is made up of atoms and molecules, and atoms are made up of nuclei and electrons, which revolve around the nucleus in a certain orbit. Just as the earth and planets like Venus and Mars revolve around the sun, each electron has a certain amount of energy and is in a certain orbit, or each orbit has a certain energy level.

Orbitals near the nucleus have lower energy levels, while those farther away have higher energy levels. The difference in energy levels between the orbitals is called the difference of energy levels. When an electron transitions from a low energy level to a high energy level, it absorbs the energy of the corresponding energy difference.

In an optical fiber, when an electron in an energy level is illuminated by a wavelength corresponding to the energy level difference, the electron in a lower energy level will jump to a higher energy level. This electron absorbs the energy of light, creating an absorption loss of light.

Silicon dioxide (SiO2), the basic material for making optical fibres, absorbs light itself, one in the ultraviolet and the other in the infrared. Currently, optical fiber communication only works in the 0, 8 ~ 1, 6 muon wavelength region, so we only discuss the loss of this working region.

The absorption peak of electron transition in quartz glass is about 0, 1 ~ 0, 2 muon wavelength in uv region. Absorption diminishes as the wavelength increases, but the area of influence is wide, until the wavelength is more than a muon. However, ultraviolet absorption has little effect on quartz fiber working in the infrared region. For example, ultraviolet absorption can reach 1dB/km in the visible light region of 0 and 6 muon wavelengths, and it can drop to 0, 2 ~ 0 and 3dB/km at 0 and 8 muon wavelengths, while only 0 and ldB/km at 1 and 2 muon wavelengths.

The infrared absorption loss of quartz fiber is caused by the molecular vibration of infrared material. There are several vibration absorption peaks in the band above 2 microns.

Due to the influence of various doped elements in the fiber, it is impossible for the quartz fiber to have a low loss window in the band of more than 2 muon, and the theoretical limit loss at the wavelength of 1 and 85 muon is ldB/km.

Through research, still discover quartz glass has a few "destroy element" in make trouble, basically be a few harmful transition metal impurity, wait like copper, iron, chromium, manganese. These "bad guys" in the light, greedy absorption of light energy, jumping around, causing the loss of light energy. By removing "troublemakers" and chemically purifying the materials used to make the fibers, losses can be greatly reduced.

Absorbed another source of silica optical fiber is hydroxyl (OH ˉ) issue of the research, it was found that hydroxyl on working in the optical fiber band have three absorption peaks, they are respectively 0, 95 mu m, 1, 1, 24 microns, and 38 microns, among them 1, 38 microns wavelength absorption loss is most serious, the impact on the optical fiber. At the wavelength of 1 and 38 microns, the absorption peak loss generated by hydroxide containing only 0 and 0001 is as high as 33dB/km.

Where did this hydroxide come from? There are many sources of hydroxyl. First, there are water and hydroxyl compounds in the materials used to make optical fiber. These hydroxyl compounds are not easy to be removed in the process of raw material purification, and finally remain in the form of hydroxyl in optical fiber. Second, there is a small amount of water in the hydrogen and oxygen materials used in fiber manufacturing. Third, water is generated by chemical reaction in the manufacturing process of optical fiber. Fourth, the entrance of the outside air brings water vapor. However, the manufacturing process has developed to such a high level that the content of hydroxide has been reduced to such a low level that the effect on the fiber is negligible.

4. Scattering loss

In the dark, shine a flashlight into the air, you can see a beam of light. Large beams of light have also been seen from overhead searchlights overnight.

So why do we see these beams of light? This is because there are many tiny particles of smoke, dust and other particles floating in the atmosphere. Light shining on these particles scatters and shoots in all directions. This phenomenon was first discovered by Rayleigh, so the scattering is called Rayleigh scattering.

How does scattering come about? It turns out that the molecules, atoms, electrons and other tiny particles that make up matter vibrate at certain natural frequencies and emit light of a wavelength corresponding to that frequency. The vibration frequency of a particle is determined by its size. The larger the particle, the lower the vibration frequency and the longer the wavelength of light emitted. The smaller the particle, the higher the vibration frequency and the shorter the wavelength of light emitted. This vibration frequency is called the natural vibration frequency of the particle. But this vibration is not self-generated, it requires a certain amount of energy. Resonance occurs when a particle is exposed to light of a certain wavelength at the same frequency as the particle's natural vibration. The electrons in the particle start to vibrate at this frequency, and the result is that the particle scatters light in all directions. The energy of the incident light is absorbed and converted into the energy of the particle, which then shoots out the energy as light energy again. Thus, to an observer on the outside, it would appear that the light, after striking the particles, is flying away in all directions.

Rayleigh scattering is also found in optical fibers, and the resulting optical loss is called Rayleigh scattering loss. In view of the current optical fiber manufacturing technology level, it can be said that Rayleigh scattering loss is inevitable. However, as the Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength, the influence of Rayleigh scattering loss can be greatly reduced when the fiber works in the long wavelength region.

5, congenital deficiency, can not help

Fiber structure is not perfect, such as bubbles, impurities, or uneven thickness in the fiber, especially the core-cladding interface is not smooth, when the light reaches these places, some light will scatter to all directions, causing loss. This loss can be overcome by improving the way optical fibers are made. Scattering causes light to be scattered in all directions. Some of the scattered light is reflected back in the direction opposite to the propagation of the optical fiber. This part of scattered light can be received at the incident end of the optical fiber. The scattering of light causes a loss of light energy, which is undesirable. However, this phenomenon can also be used to our advantage, because if we analyze the strength of the received beam at the sending end, we can detect the break point, defect and loss of the fiber. Thus, through human intelligence, bad things become good things.

Fiber loss

Optical fiber loss in recent years, optical fiber communication has been widely used in many fields. In order to realize optical fiber communication, an important problem is to reduce the loss of optical fiber as much as possible. The so-called loss is the attenuation of the fiber per unit length, which is dB/km. The loss of optical fiber directly affects the transmission distance or the distance between relay stations. Therefore, it is of great practical significance for optical fiber communication to understand and reduce the loss of optical fiber.

Absorption loss of optical fiber

This is caused by the absorption of optical energy by optical fiber materials and impurities. They consume the optical energy in the form of thermal energy in the optical fiber, which is an important loss in optical fiber loss. Absorption loss includes the following:

Intrinsic absorption loss this is the loss caused by the intrinsic absorption of a substance. It has two bands, one in the near infrared region of 8 to 12 microns, where the intrinsic absorption is due to vibration. Another material's natural absorption band is in the ultraviolet band, and when it's absorbed strongly, its tail will go to the 0, 7, 1, 1 muon band.

2. Absorption loss caused by dopant and impurity ions in fiber materials contains transition metals such as iron, copper and chromium, which have their own absorption peaks and absorption bands and vary with their valence states. The fiber loss caused by the absorption of transition metal ions depends on their concentration. In addition, the presence of OH- also produces absorption loss. The basic absorption peak of OH- is near 2, 7 muon, and the absorption band is in the range of 0, 5 ~ 1, 0 muon. For pure quartz fiber, the loss caused by impurity can not be considered.

3, atomic defect absorption loss optical fiber materials due to heat or strong radiation, it will be excited to produce atomic defects, resulting in the absorption of light, loss, but in general this effect is very small.

The scattering loss of optical fiber

The internal scattering of optical fiber will reduce the transmission power and produce loss. Rayleigh scattering is the most important part of optical fiber scattering.

During the heating process of fiber materials, due to thermal disturbance, the compressibility of atoms is uneven, the density of materials is uneven, and the refractive index is uneven. This unevenness, which is fixed during cooling, is smaller than the wavelength of light. When light is transmitted, it encounters these inhomogeneous materials which are smaller than the wavelength of light wave and have random fluctuations, which change the transmission direction, produce scattering and cause loss. In addition, uneven oxide concentration and doping in the fiber will also cause scattering and loss.

Waveguide scattering loss

This is the scattering caused by random distortion or roughness of the interface. In fact, it is the mode conversion or mode coupling caused by surface distortion or roughness. One mode will generate other modes of transmission and radiation due to the fluctuation of the interface. Due to the various patterns in the optical fiber transmission attenuation is different, in the process of long distance mode conversion, small attenuation model into attenuation model, after continuous transform and inverse transform, although each mode would lose balance, but overall generate additional loss model, namely due to the conversion generated additional loss model, the additional loss is the scattering loss. To reduce this loss, it is necessary to improve the optical fiber manufacturing process. This loss can be largely ignored for well - drawn or high - quality optical fibers.

4. Radiation loss caused by fiber bending

Fiber optics are soft and can be bent, but when bent to a certain extent, the fiber can conduct light, but it will change the way the light is transmitted. By converting from transmission mode to radiation mode, a part of the light energy penetrates into the cladding or passes through the cladding, resulting in radiation mode leakage and loss. When the bending radius is larger than 5 ~ 10cm, the loss caused by bending can be ignored.